1. Field of the Invention
The present invention relates to a light-emitting diode for use as a light source in a photocoupler, remote control or other such application requiring high-output infrared light.
2. Description of the Prior Art
There are currently known infrared light-emitting diodes such as the Si-doped GaAlAs type described on pages 2485 to 2492 of the Journal of Applied Physics, Vol. 48, No. 6, Jun. 1977, illustrated in FIG. 1(a). It is fabricated by a method comprising epitaxially growing GaAlAs layers on a GaAs single crystal substrate 4 by contacting a Si-doped GaAlAs solution with the substrate, cooling the solution. Selecting appropriate growth temperatures at this time makes possible the continuous formation of an N-type Ga.sub.1-x Al.sub.x As layer 1 and a P-type Ga.sub.1-y Al.sub.y As layer 2 using a single solution. The reason for this is that at a high temperature, Si, a group IV element, enters the GaAlAs single crystal at a group III element Ga (Al) site, while at a low temperature the entry takes place at a group V element As site. Use of a ramp cooled growth method is economical, as it allows the number of solutions to be reduced. Moreover, because this method does not require that the PN junction faces be exposed to an atmosphere in a furnace, there are fewer crystal defects. Light-emitting diodes are processed from this epitaxial wafer by etching away the GaAs substrate with a suitable etchant.
With the ramp coiled growth method, the Al concentration in each layer in the direction of layer growth is not constant, but decreases as growth proceeds from the initiation point to the completion point. The reason for this is that, since the segregation coefficient of Al is greater than 1, the concentration of Al in the solution decreases during layer growth formation. Also, the higher the Al concentration W is, the larger the bandgap of the Ga.sub.1-w Al.sub.w As mixed crystal. Thus, as shown in FIG. 1(b), Al concentration, that is bandgap, in the light-emitting diode of. FIG. 1(a) is a constant decrease from the growth initiation point of the first layer to the growth termination point of the second layer. In the first layer the Al concentration is highest at the surface (X1) and lowest at the interface between the first and second layers (X2). In the second layer, the Al concentration is highest at the interface between the first and second layers (Y1) and lowest at the surface (Y2). To enable the first and second layers to be continuously grown using a single solution, at the interface between the first and second layers the Al concentration in both layers is the same (X2=Y1).
Light emitted by the light-emitting diode of FIG. 1(a) is produced mainly by electrons injected from the PN junction into the P-type second layer recombining with holes. The energy of the emitted light is more or less equal to the crystal bandgap at the recombination site. Generally the light is of a type whereby it is absorbed by crystal having a smaller bandgap than the energy of the light. Therefore, the first layer of the light-emitting diode of FIG. 1(a) is transparent to the emitted light. In the second layer, regions 9 and 10 on the surface side of the second layer relative to the radiative recombination center 7 at which light is emitted by recombination are absorbing portions, and the interface side 8 between the first and second layers is transparent.
In the conventional light-emitting diode shown in FIG. 1(a), the regions 9 and 10 between the radiative recombination center 7 and the surface of the second layer are light absorbing portions. Even within the absorbing portions, the closer the portion in the region 10 is to the surface, the lower the Al concentration and the higher the light absorption factor.
To suppress light absorption, methods of increasing the external light output efficiency have been proposed comprising providing over the GaAlAs p-n junction a P-type GaAlAs window layer having a higher bandgap energy than the P-type GaAlAs constituting the emission layer (Japanese Patent Public Disclosures SHO 60-206184 and HEI 6-21507).
Each of these utilizes the spontaneous inversion of Si to form the PN junction, over which a P-type GaAlAs window layer with a bandgap that is larger than that of the P-type GaAlAs emission layer is formed, with the Al concentration in each layer being limited to within a prescribed range so as to raise the emission efficiency. Each also uses the emitted light passing through the P-type GaAlAs window layer.
However, while conventional light-emitting diodes such as these are used in the light-emitting sections of remote controls and the like, there is a market demand for higher-power products.
Thus, the object of the present invention is to provide a light-emitting diode having a high emission efficiency and a low forward voltage in which output is improved by suppressing the absorption of emitted light by epitaxially grown layers.